Patent Document 1 discloses a clamping mechanism of a rotary table device. The clamping mechanism clamps a rotary shaft, to which a circular table is secured, by a clamp disc (brake plate) and a piston serving as clamping means. The clamp disc is mounted to the rotary shaft so as to be incapable of rotating relative to the rotary shaft. The piston is normally urged towards the clamp disc by a coil spring (compression spring).
In the disc type clamping device described above, in order to set the clamp disc in a state in which it is clamped by a piston, that is, in order to set the clamp disc in a state in which it is held so as to be incapable of rotating, it is necessary for the piston to be in a state in which it is incapable of rotating with respect to a housing (frame body). That is, it is necessary for the piston to be provided so that it is capable of moving in an axial direction of the rotary shaft (urging direction towards the clamp disc) and so that it is incapable of rotating relative to the housing in a rotation direction of the rotary shaft.
Accordingly, in the type in which the clamp disc is clamped by displacing the piston with respect to the housing in the axial direction of the rotary shaft, the piston needs to have a structure that stops its rotation, thereby complicating the internal structure of a rotary table device and increasing manufacturing costs.
For example, a structure that slides and guides the piston in the axial direction of the rotary shaft by bringing the piston into contact with a surface of the housing in a direction intersecting with the rotation direction of the rotary shaft and a structure in which the piston is slid and guided in the axial direction of the rotary shaft by a shaft secured to the housing and passed through the piston are available as the aforementioned structure for stopping the rotation. However, in these structures, friction is generated between the piston and a sliding-guiding surface through long-term use, and rattling between the piston and the sliding-guiding surface is increased. As a result, the clamped state cannot be stably maintained. In particular, if a force acts upon the circular table when processing a workpiece, displacement occurs with respect to an indexed angular position in correspondence with the rattling, thereby reducing indexing precision (processing precision).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-95668